1. Field of the Invention
The present invention relates to feedthroughs for optical waveguides, and more particularly, to hermetically sealed feedthroughs suitable for use in high pressure, high temperature, and/or other harsh environments.
2. Description of the Related Art
In many industries and applications, there is a need to have small diameter wires or optical waveguides penetrate a wall, bulkhead, or other feedthrough member wherein a relatively high fluid or gas differential pressure exists across the feedthrough member. In addition, one or both sides of the feedthrough member may be subjected to relatively high temperatures and other harsh environmental conditions, such as corrosive or volatile gas, fluids and other materials.
In the case of electrical systems, these devices, called feedthroughs or penetrators, typically are constructed by using metal ‘pins’ exhibiting high conductivity and a low thermal coefficient of expansion. The pins are concentrically located within a hole in a housing, and the resulting annular space is filled with a suitable sealing glass or other material. Critical to the success of such seals is the selection of the metal housing, sealing glass, and electrical pin to ensure completion of a compression seal around the ductile inclusion (pin). As the operating temperature range of the feedthrough increases, the control of thermal expansion rates becomes increasingly important in order to avoid failure of the feedthrough by excessive thermal stress at the interface layers between the various materials. This technology is relatively mature for electrical feedthroughs, and commercial devices are readily available that meet service temperatures in excess of 200° C.
More recently, with the introduction of optical sensors, particularly sensors for use in oil and gas exploration and production and for life in harsh industrial environments, a need has emerged for a bulkhead feedthrough that can seal an optical fiber at high pressures of 20,000 psi and above, and high temperatures of 150° C. to 300° C., with a service life of 5 to 20 years. An exemplary sensing assembly for use in harsh environments is disclosed in U.S. Pat. No. 6,439,055, which issued on Aug. 27, 2002, entitled “Pressure Sensor Assembly Structure To Insulate A Pressure Sensing Device From Harsh Environments”, which is assigned to the Assignee of the present application and is incorporated herein by reference in its entirety.
There are several problems associated with constructing such an optical fiber feedthrough. One of these problems is the susceptibility of the glass fiber to damage and breakage. This is due to the flexibility of the small size fiber, the brittle nature of the glass material, and the typical presence of a significant stress concentration at the point where the fiber enters and exits the feedthrough. Attempts to use a sealing glass, such as that used with electrical feedthroughs, have had problems of this nature due to the high stress concentration at the fiber-to-sealing glass interface.
Another problem with sealing an optical fiber, as opposed to sealing a conductive metal “pin” in an electrical feedthrough, is that the fused silica material of which the optical fiber is made, has an extremely low thermal expansion rate. Compared to most engineering materials, including metals, sealing glasses, as well as the metal pins typically used in electrical feedthroughs, the coefficient of thermal expansion of the optical fiber is essentially zero. This greatly increases the thermal stress problem at the glass-to-sealing material interface.
One technique used to produce optical fiber feedthroughs is the use of a sealed window with an input and an output lensing system. In this technique, the optical fiber must be terminated on each side of a pressure-sealed window, thus allowing the light to pass from the fiber into a lens, through the window, into another lens, and finally into the second fiber. The disadvantages associated with this system include the non-continuous fiber path, the need to provide two fiber terminations with mode matching optics, thus increasing manufacturing complexity and increasing the light attenuation associated with these features.
It is often desirable to mount fiber optic based sensors in harsh environments that are environmentally separated from other environments by physical bulkheads. An exemplary such fiber optic based sensor is disclosed in co-pending U.S. patent application Ser. No. 09/205,944, entitled “Tube-Encased Fiber Grating Pressure Sensor” to T. J. Bailey et al., which is assigned to the Assignee of the present invention and is incorporated herein by reference in its entirety. This exemplary optical sensor is encased within a tube and certain embodiments are disclosed wherein the sensor is suspended within a fluid. The sensor may be used in a harsh environment, such as where the sensor is subjected to substantial levels of pressure, temperature, shock and/or vibration. In certain environments, such sensors are subjected to continuous temperatures in the range of 150° C. to 250° C., shock levels in excess of 100 Gs, and vibration levels of 5G RMS at typical frequencies between about 10 Hz and 2000 Hz and pressures of about 15 kpsi or higher.
However, as discussed above, the harsh environments where the sensors are located generally must be isolated by sealed physical barriers from other proximate environments through which the optical fiber communication link of the sensor must pass. It is important to seal the bulkhead around the optical fiber to prevent adjacent environments in the sensor from contaminating the optical fiber communication link. If the optical communication fiber is compromised by contamination from an adjacent harsh sensor environment, the optical fiber and all sensors to which it is connected are likely to become ineffective.
There is a need therefore, for an optical waveguide feedthrough assembly capable of operating in relative high temperature and high pressure environments.